Optical fibers and gratings are useful for telecommunication transmission and networking. Optical gratings are useful in controlling the paths or properties of traveling light. Gratings based on optical fibers are of particular interest as components in modern telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting information-containing optical signals over long distances with low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. Typical optical fibers are made of high purity silica, and various concentrations of dopants may be added to control the index of refraction.
In long-distance transmission of optical signals, the accumulation of signal dispersion may be a serious problem. This problem intensifies where there is an increase in the distance the signals travel or the number of channels in a wavelength-division-multiplexed (WDM) optical communication system. Efforts to compensate for chromatic dispersion to date have involved use of dispersion compensating fibers, dispersion compensating gratings, or a combination of both. See M. I. Hayee et al, IEEE PHOTONICS TECHNOLOGY LETT., Vol. 9, No. 9, p. 1271 (1997); R. I. Laming et al., IEEE PHOTONICS TECHNOLOGY LETT., Vol. 8, No. 3 (1996); W. H. Loh et al., IEEE PHOTONICS TECHNOLOGY LETT., Vol. 8, No. 7 (1996); K. O. Hill et al., OPT. LETT., Vol. 19, p. 1314 (1994); and U.S. Pat. No. 5,701,188 issued to M. Shigematsu et al., on Dec. 23, 1997, incorporated herein by reference. These dispersion compensating devices, however, are not flexible and provide only a fixed degree of compensation for chromatic dispersion.
Optical gratings are important elements for selectively controlling specific wavelengths of light transmitted within optical communication systems. Such gratings may include Bragg gratings, long-period gratings, and diffraction gratings. These gratings typically comprise a body of material with a plurality of spaced-apart optical grating elements disposed in the material. Often, the grating elements comprise substantially equally-spaced index perturbations, slits, or grooves, but unequally-spaced (chirped) gratings are used as well.
For all types of gratings, it would be highly useful to be able to reconfigure or tune the grating to selectively adjust the controlled wavelengths. A difficulty with conventional Bragg gratings is that they filter light of only a fixed wavelength. Each grating selectively reflects light in a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However, in many applications, such as wavelength division multiplexing (WDM), it would be desirable to have a grating whose wavelength response can be controllably altered. Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with low back reflections. A difficulty with conventional long-period gratings, however, is that their ability to dynamically equalize amplifier gain is limited, because they filter only a fixed wavelength acting as wavelength-dependent loss elements. Each long-period grating with a given periodicity (.lambda.') selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle will have a different spectral content depending on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed.
One attempt to make a tunable waveguide grating involves applying strain to the grating using a piezoelectric element. See Quetel et al., 1996 Technical Digest Series, Conf. on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, paper No. WF6. A difficulty with this approach is that the strain produced by piezoelectric actuation is relatively small which limits the tuning range of the device. Moreover, this approach requires that electrical power be continuously applied at relatively high voltage, e.g., approximately 100 volts. Other tunable gratings involving the application of strain to the grating are disclosed in U.S. Pat. application Serial No. 08/791,081, now U.S. Pat. No. 5,781,677, filed by Jin et al. on Jan. 29, 1997, U.S. Pat. application Ser. No. 09/020,206, bending, filed by Espindola et al. on Feb. 6, 1996, U.S. Pat. application Ser. No. 08/971,956, bending, filed by Jin et al. on Oct. 27, 1997, and U.S. Pat. application Ser. No. 08/971,953, now U.S. Pat. No. 5,957,574, filed by Fleming et al. on Oct. 27, 1997, all of which were assigned to the present assignee and are incorporated herein by reference. The use of magnetostriction for grating chirping was disclosed in J. L. Cruz el al., ELECTRONIC LETT. 33(3), p. 235 (1997), but the approach there did not demonstrate programmable or latchable magnetostrictive strain and did not encompass a dispersion compensating device.
As may be appreciated, those concerned with technologies involving optical communication systems continue to search for new devices and methods to selectively control and filter transmitted wavelengths and compensate for chromatic dispersion. In particular, it would be advantageous to have a device for programmably shifting wavelengths that does not require the continuous application of power. This invention discloses a magnetostrictively tunable and latchable device for shifting wavelengths and compensating for dispersion, and optical communication systems comprising such a device.